Overhaul regression testing: e2e suite, fuzzing, CI, and bug fixes

Add a protocol-level end-to-end suite (in-process fully wired server on
an ephemeral port pair, driven by hotline.Client over TCP) covering
handshake, login, public and private chat, message board, threaded
news, file list/download/upload, account admin, disconnect
notification, and shutdown broadcast. The harness retries on a fresh
port pair when another process steals a probed port before
ListenAndServe binds it, and Server gains WithConnectionRateLimit so
tests can disable the per-IP connection throttle.

Add native fuzz tests for Transaction, Field, and flattened file
object decoding, and fix the bugs the new tests surfaced:

- Transaction.Write panicked on out-of-range attacker-controlled size
  fields, and transactionScanner's uint32 length addition could wrap
  and yield a truncated token. The information fork size declared in
  an untrusted fork header is now bounded too.
- Client keepalive read c.done unsynchronized while Disconnect
  replaces it under the mutex.
- The shared Agreement's Seek+ReadAll login path raced concurrent
  logins; the server now prefers an AgreementBytes() snapshot.

Fill unit-test gaps (main's config-copy helpers, file resume data,
ReloaderFunc, R2 error injection and env validation) and add a CI test
workflow (build/vet + race-enabled shuffled suite), fixed lint
workflow triggers with golangci-lint v2.6, and Makefile test/cover/
lint/fuzz targets.

1 files changed, 8 insertions, 4 deletions

diff --git a/hotline/client.go b/hotline/client.go
index 23bfe9c..b09a01d 100644
--- a/hotline/client.go
+++ b/hotline/client.go
@@ -84,15 +84,19 @@ func (c *Client) Connect(address, login, passwd string) (err error) {
 		return fmt.Errorf("error sending login transaction: %w", err)
 	}
 
-	// start keepalive go routine
-	go func() { _ = c.keepalive() }()
+	// start keepalive go routine.  Capture the done channel now so keepalive never races with
+	// Disconnect, which replaces c.done under the mutex.
+	c.mu.Lock()
+	done := c.done
+	c.mu.Unlock()
+	go func() { _ = c.keepalive(done) }()
 
 	return nil
 }
 
 const keepaliveInterval = 300 * time.Second
 
-func (c *Client) keepalive() error {
+func (c *Client) keepalive(done <-chan struct{}) error {
 	ticker := time.NewTicker(keepaliveInterval)
 	defer ticker.Stop()
 
@@ -100,7 +104,7 @@ func (c *Client) keepalive() error {
 		select {
 		case <-ticker.C:
 			_ = c.Send(NewTransaction(TranKeepAlive, [2]byte{}))
-		case <-c.done:
+		case <-done:
 			return nil
 		}
 	}